1. Field of the Invention
This invention relates to a heater construction for use as a heat source of a hot isostatic pressing apparatus, and more particularly to a heater having a plurality of heater units mechanically connected with each other by a connecting means which has excellent electric insulating properties under high temperature and pressure conditions along with sufficient resistance to thermal impacts, ensuring stable heating operation by the heater over a long time period.
2. Description of the Prior Art
The hot isostatic pressing apparatus which heats and compresses a workpiece or workpieces in a high temperature and pressure gas atmosphere mostly employs a heater of the construction shown in FIG. 1(a) to (c), with heating elements of special material like graphite to ensure endurance under severe operating conditions of the hot isostatic pressing (hereinafter referred to simply as "HIP" for brevity) at a temperature higher than 1250.degree. C. and a pressure of from several hundreds to several thousands atms.
The heating element of such non-metallic material has a high compression strength but, if formed with a reduced thickness in order to increase the value of resistance, it is easily flexed and readily damaged even by a slight mechanical shock due to its fragility. Therefore, it becomes necessary to take utmost care in handling and in fixation in a HIP furnace of high temperature and pressure.
The conventional heater of this sort has the so-called segment type heating element 3b of U- or W-shape 3c as shown in FIG. 1(b) or of rod shape as shown in FIG. 1(c). The segments of these shapes are relatively easy to manufacture but usually necessitate the troublesome work of connecting the individual segments within a furnace. In contrast, the unitary heating element 3a shown in FIG. 1(a) is easier to assemble into a furnace. In any event, the respective heating elements 3a, 3b and 3c have merits and demerits, and, as far as the accuracy of assembling into the furnace is concerned, the segment type is difficult to build with an accurate cylindrical shape as compared with the unitary type. This is reflected by the fact that the unitary type is more favorably accepted in most cases.
In order to cope with the recent trend toward HIP systems of larger scales, the heater elements 3' have come to be used in a stacked form as shown particularly in FIG. 2, stacking more than two heating element units 3' one on another to form a heater 2'. This is because more difficulties are involved in the manufacturing process of the unitary type heaters of larger sizes, and it is necessary to divide the heater 2' into a plurality of heating zones of different capacities for eliminating temperature variations between the upper and lower localities within the furnace. The heater structure with multiple stories can be obtained simply by fixing and connecting the stacked heating element units 3' usually with the use of annular insulators 5'.
However, the heaters 2' as required by the large HIP apparatus have problems in that there is a possibility of damaging either the insulator 5' or the heating element units 3' whichever is lower in strength by the large thermal stress due to the differences in thermal expansion, and in that selection of insulators 5' which have good electric insulating property at high temperatures as well as satisfactory resistance to thermal shocks will invite a disadvantage in production costs, and technical difficulties in the manufacturing process.